Methodology

May 23, 2009 3:16 AM

The Times studied DWI arrest data from Jan. 1, 2003, through March 2009, gathered from the Shreveport and Bossier City police departments, and Caddo and Bossier sheriff's offices. While efforts were made to ensure only DWI arrests were included, the data could contain some arrests made as a result of failure to appear in court on an existing DWI charge.

The newspaper also studied conviction data from Jan. 1, 2003, through March 2009, obtained from the Shreveport City Court and Caddo District Attorney. The conviction data includes only those cases that are completed.

In analyzing conviction data to determine the percentage of Shreveport police arrests appearing to not be prosecuted, using an offender's birthdate, the newspaper matched arrests against convictions from court and district attorney records. Those results were validated by a random sample offering a 95 percent reliability rating and a 5 percent margin of error, plus or minus.

Even so, the resulting analysis could include some offenders whose cases were prosecuted but because of clerical error, such as an incorrect birth date, or other factors, such as a pending court date, were not excluded from the findings.Multiple efforts to obtain conviction data from Bossier City Court and the Bossier Parish Court were unsuccessful.